Since the optical output power and the extinction ratio of a laser diode vary depending upon the temperature, some control is required for the temperature variation in order to implement desired optical output power and extinction ratio. Normally, a laser diode emits modulation light by receiving bias current and modulation current input thereto through a laser driver circuit. The optical output power and the extinction ratio of the laser diode are determined depending upon the bias current and the modulation current. Accordingly, in order to fixedly maintain the optical output power and the extinction ratio, the optical output power is feedback controlled and the modulation current is controlled to a predetermined value. Such control as just described is known APC (Auto Power Control) control.
In Patent Document 1, modulation current to be input to a laser diode is varied in response to a temperature characteristic of the laser diode. Consequently, temperature compensation is carried out under a low-temperature environment or a high-temperature environment.
[Patent Document 1] Japanese Patent Laid-Open No. 2002-246687